1. Field of the Invention
The present invention relates to a vibration sensing device or an accelerometer used as a knock sensor which converts knocking vibration of an engine into an electric signal by a vibration-voltage converting element.
2. Related Art
Conventional knock sensors for engines generally have a sensor housing attachable to a cylinder block of an engine, a vibration plate housed in the sensor housing, a piezoelectric element joined to the vibration plate and an output terminal (connector terminal) electrically connected to the piezoelectric element. The piezoelectric element produces a voltage signal corresponding to the knocking vibration occurring in the sensor housing and transmitted to the vibration plate. It is proposed by JP-U 62-128331 that, for simplifying the electric connection between the piezoelectric element and the output terminal, a leaf spring-type resilient conductive member is used. The top end part of the resilient conductive member fixed to the output terminal side is held in resilient contact with the electrode surface of the piezoelectric element so that the voltage signal from the piezoelectric element is transmitted to the output terminal through the resilient conductive member.
It may be effective to set the resiliency (spring constant or elastic constant) of the resilient conductive member large so that the contacting pressure of the resilient conductive member to the piezoelectric element is increased, thereby enhancing the reliability of electric contact between the piezoelectric element and the resilient conductive member. Such a large spring constant, however, is likely to cause damage such as cracks in the piezoelectric element because of the excessive pressing force on the piezoelectric element, when the resilient conductive member is pressed onto the piezoelectric element to flex at the time of assembling the two. Even if not damaged at the assembling process, the piezoelectric element may possibly wear excessively.
Therefore it is desired to set the spring constant of the resilient conductive member low for protecting the piezoelectric element from damages. The lower spring constant will result in lower contacting pressure of the resilient conductive member, causing the resilient conductive member to resonate with the knocking vibration. That is, as the spring constant of the resilient conductive member and the resonance frequency are in proportional relation to each other, the resonance frequency becomes lower with the decrease in the spring constant and often as low as below 10 KHz which is close to the resonance frequency of the vibration plate (piezoelectric element). This resonance of the vibration plate will also cause the resilient conductive member to resonate as well at the time of occurrence of the knocking vibration. The contact part of the resilient conductive member temporarily bounces from the piezoelectric element, resulting in temporary interruption of the electric connection. As a result, not only the knock signal will not be provided accurately, but also the fixed part of the resilient conductive member will break because of fatigue.